1) BYU isn't likely to open up the books and let anyone see the info to report on this

2) BYU does take from students as well. At last report something like 14% of their athletic budget comes from student tuition. Being private means BYU doesn't have to break down what all the tuition is for, but it's clear some it is goes to athletics.

3) BYU (and the LDS church) likely wouldn't want this because it could raise the uncomfortable issue of tithing money going to athletics. While this doesn't happen directly, it's not as far removed as some would like. Tithing is used to discount tuition for students, which is then increased to pay for athletics.